This invention is related generally to electrodes for monitoring cortical electrical activity in order to define epileptogenic foci. However, the invention is not limited to monitoring brain electrical activity but also has improved features for monitoring and electrical stimulation of brain and other tissue.
Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various methods have been used in attempting to determine epileptogenic foci, and all involve sensing of cortical electrical activity using electrical contacts applied in various ways.
Standard scalp contacts have been used for many years, but accurate localization is usually very difficult with recordings obtained from such contacts. Therefore, many epilepsy centers in recent years have used intracranial recording techniques to better define regions of cortical epileptogenicity.
Intracranial recording techniques have used either of two different types of electrodes--intracortical depth electrodes or subdural strip electrodes. The far more commonly used technique of intracranial recording uses intracortical depth electrodes, but other techniques using subdural strip electrodes, first utilized many years ago, have been shown to be relatively safe and valuable alternatives.
The relative safety of subdural strip electrodes lies in the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures with contact bands spaced along their lengths. Such electrodes are inserted into the brain in order to establish good electrical contact with different portions of the brain. These electrodes must be stiff in order to penetrate brain tissue. Subdural strip electrodes, on the other hand, are generally flat strips supporting contacts spaced along their lengths. Such strip electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain.
A typical subdural strip electrode of the prior art is shown in FIGS. 1-4 and is disclosed in U.S. Pat. No. 4,735,208. The '208 patent discloses a subdural strip electrode 10 having an elongated flexible silicone dielectric strip 14, a plurality of spaced aligned flat electrical stainless steel contact disks 16 held within dielectric strip 14, and lead wires 18 exiting strip 14 from a proximal end 20 thereof.
Dielectric strip 14 of strip electrode 10 has front and back dielectric layers 22 and 24, respectively. Each front layer 22 has a front layer opening 26 for each contact disk 16. Openings 26 are circular and somewhat smaller in diameter than contact disks 16. Front and back layers 22 and 24 are sealed together by adhesive and/or heat such that they form, in essence, an integral dielectric strip.
The dielectric strips are generally made of a flexible, non-conductive material in order to allow the subdural strip electrode to conform to the surface of the brain. For this reason, metals are generally inappropriate for the dielectric strip.
The subdural strip electrodes of the prior art are predominately rectangular in cross-section. Other subdural strip electrodes of the prior art have a circular or round cross section.
The lead wires are generally routed out through a stab wound in the skin remote from the electrode and generally terminate in a distal end with ring-type terminals. These ring-type terminals are then connected to monitoring equipment by means of a connector. Such a prior connector is disclosed in U.S. Pat. No. 4,869,255.
The structure of these prior art electrodes results in high manufacturing costs. Generally, the contact disks and lead wires are manually placed onto one of the dielectric strips; the second dielectric strip is placed over the contact disks and lead wires; and the two dielectric strips are sealed together. Because the location of the contact disks within the brain must be precisely known in order to determine epileptogenic foci, the contact disks must be precisely placed along the dielectric strip. The lead wires are thin and may easily tangle with each other or may be twisted or broken during the assembly process. Therefore, careful quality control of the assembly process is necessary.
Additionally, the thin lead wires may have a high electrical resistance, causing problems with recording brain electrical activity.
After lead wires exit the proximal end of the electrode, they are no longer supported by the flexible electrode matrix and may break.
An additional manufacturing step is necessary to manufacture the ring-type terminals.
There is a need for an improved implantable electrode which may be manufactured with a printed circuit to lower manufacturing costs and improve quality control.
Flexible printed circuits have been used for some time in electronic devices such as liquid crystal display tubes, ECD and solar cells for mechanically and electrically connecting electrode portions of the electronic devices and a printed circuit board. Such flexible printed circuits are discussed in U.S. Pat. No. 5,493,074 and U.S. Pat. No. 5,569,886, herein incorporated by reference.
U.S. Pat. No. 4,461,304 discloses a microelectrode for insertion into the brain with parts of the microelectrode formed of a printed circuit. However, this electrode is not suitable for use as a subdural strip electrode because the electrode is not flexible and the substrate is a metal. Furthermore, the problem addressed by the '304 patent is different from the problem being addressed in this application. The '304 patent addresses the problem of recording the electrical potentials of individual neurons of the brain. The current patent application addresses the problem of recording brain electrical activity at a much grosser level, that is, at epileptogenic foci, which may comprise thousands or tens of thousands of neurons.